I found that the better I do the prep work, the better the result will be and squaring falls in the prep work. On a simple block, I at least measure the blocks so I know how much - if any - the blocks are off. Anything too large gets trimmed and if the difference isn't too much, I use the smaller ones as is (adjusting the seam allowance as I go). I think that comes with experience. I used to be way more retentive in some detail work.